The present invention relates to the return of oil from the evaporator to the compressor in a refrigeration chiller. With more particularity, the present invention relates to the return of oil from the evaporator to the compressor in a refrigeration chiller using an eductor. With still more particularity, the present invention relates to the use of an eductor to return oil from the evaporator to the compressor in a screw compressor-based refrigeration chiller where the fluid by which the eductor is powered and by which the return of oil from the evaporator is motivated is oil sourced from the chiller's oil separator, enroute to its use in the compressor.
Oil migration from the compressor in a refrigeration chiller into the chiller's refrigeration circuit occurs in virtually all compressor-driven chiller systems. The return of such oil, from the chiller's evaporator where it tends to accumulate, is an age old problem.
Many and varied techniques for returning such lubricant from the evaporator back to the compressor in a refrigeration chiller exist. Among the many oil return arrangements and apparatus used for such purposes are systems based on the use of an eductor which draws oil-rich liquid from the system evaporator and delivers it back to the system compressor. The motive force for such eductors is most typically provided by high pressure system refrigerant. Because high pressure system refrigerant is diverted from the refrigeration circuit to achieve oil return in such systems, the overall efficiency of the chiller system is penalized because such gas will have to be recompressed without having been used to cool the load which it is the purpose of the chiller to cool.
More recently, newer, lower pressure refrigerants have come to be used in refrigeration chiller systems with the result that pressure differentials within the chiller and between the condenser and evaporators are not so large as in previous systems. Such pressure differentials may be insufficient to ensure that a refrigerant-powered eductor will reliably cause the return of oil from the system evaporator to the system compressor under all system operating conditions.
Further, with the advent of use of compressors of the so-called screw type, additional uses for system lubricant, such as for sealing and cooling purposes within the compressor, have evolved which inherently cause a relatively large amount of lubricant to become entrained in the refrigerant gas stream that is discharged from the compressor. This, in turn, requires the use of an oil separator downstream of the compressor to capture such oil, prior to its being carried into the system's condenser, and to facilitate its return to the compressor.
Still further, with the advent of commercial use of evaporators of the so-called falling film type in refrigeration chillers, the importance of oil return and the complexities associated therewith have been exacerbated. The difficulties and complexity of achieving oil return in such systems is discussed in U.S. Pat. 5,761,914, assigned to the assignee of the present invention and which is incorporated herein by reference. That patent illustrales one current state of the art process and the apparatus associated with it for assuring oil return from the evaporator in a refrigeration chiller which is screw compressor-driven and which employs a falling film evaporator. As will be appreciated from a study of the '914 patent, the oil return apparatus/methodology disclosed therein, while efficient, is somewhat complicated, adds expense to the chiller and requires the use of controls and valving to accomplish oil return.
The need continues to exist for a simplified yet reliable and efficient arrangement by which to return oil from the evaporator to the compressor in a screw compressor-driven refrigeration chiller which, by its nature, does not penalize the operating efficiency of the chiller to any great extent, which will operate to return oil to the compressor under all system operating conditions and which, by its nature, is relatively simple and passive in that it does not require the use of valves or controls to achieve oil return.